1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device suitable to power electronics and other applications.
2. Background Art
The ON resistance of a vertical power MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) greatly depends on the electric resistance of its conduction layer (drift layer). The dopant concentration that determines the electric resistance of the drift layer cannot exceed a maximum limit, which depends on the breakdown voltage of a p-n junction formed by the base and the drift layer. Thus there is a tradeoff between the device breakdown voltage and the ON resistance. Improving this tradeoff is important for devices with low power consumption. This tradeoff has a limit determined by the device material. Overcoming this limit is the way to realizing devices with low ON resistance beyond existing power devices.
As an example MOSFET for solving this problem, a structure is known as a “superjunction structure”, which is formed by p-type pillar regions and n-type pillar regions buried in the drift layer. In the superjunction structure, a non-doped layer is artificially produced by equalizing the amount of charge (amount of dopant) contained in the p-type pillar region and the n-type pillar region. Thus, with holding high breakdown voltage, a current is allowed to flow through the highly doped n-type pillar region, thereby realizing low ON resistance beyond the material limit. For holding high breakdown voltage, it is necessary to accurately control the amount of dopant in the n-type pillar region and the p-type pillar region.
In such a MOSFET with a superjunction structure formed in the drift layer, the design of the edge termination structure is also different from that of conventional power MOSFETs. Because the edge termination section as well as the device section needs to hold high breakdown voltage, the superjunction structure is formed also in the edge termination section. In this case, when the amount of dopant in the n-type pillar region is equal to that in the p-type pillar region, the breakdown voltage of the edge termination section decreases more significantly than that of the device section (cell section). JP 2000-277726A proposes a structure for preventing the decrease of breakdown voltage of the edge termination section where the edge termination section is formed from a high-resistance layer without a superjunction structure.
However, in this structure, the superjunction structure is discontinuous between the device section and the edge termination section. In the outermost portion of the superjunction structure, the dopant concentration in the p-type pillar region or n-type pillar region must be decreased to about half that in the cell section. For realizing such dopant concentration in the pillar region varied with position, the dose amount of ion implantation must be varied with position, or the opening width of the implantation mask must be varied. Varying the dose amount with position leads to decreased throughput such as implantation being divided into twice. On the other hand, varying the mask width can be easily realized by varying the lithography mask width. However, a conversion difference occurs between the lithography mask and the resist mask used for actual implantation. Dispersion in this conversion difference is equivalent to dispersion in the amount of dopant. Thus, unfortunately, the edge termination structure promising for high breakdown voltage in principle is difficult to realize and susceptible to process dispersion.
In the edge termination section without the “superjunction structure”, the depletion layer extends vertically and horizontally. Hence the electric field concentrates at the edge of the base connected to the source electrode. Even if a guard ring structure or a field plate structure is used for preventing electric field concentration at the base edge, electric field concentration occurs at the edge of the guard ring layer or the edge of the field plate electrode in the semiconductor layer of the edge termination section.
Even if a high breakdown voltage can be held by forming a high-resistance layer in the edge termination section without the “superjunction structure”, a sharp electric field peak may occur in the edge termination section. In this case, hot carriers generated by the high electric field degrades the field insulating film, and is likely to cause reliability degradation such as leak current variation, breakdown voltage variation, and breakdown. Furthermore, if an avalanche breakdown occurs in the edge termination section during application of high voltage, carriers due to the avalanche current further increases the electric field peak, and unfortunately, is likely to cause current concentration and device breakdown. Hence it is difficult to achieve high avalanche withstanding capability. Also in the recovery state after operation of the body diode, the vicinity of the base edge in the edge termination section is carrier-rich. Hence, with a high electric field peak, a local avalanche breakdown occurs, and is likely to cause device breakdown. Thus it is difficult to achieve high recovery capability.
In a structure disclosed in JP 2000-183350A, the edge termination section has a superjunction structure where the depth of p-type pillar regions is varied stepwise. However, in JP 2000-183350A, the edge termination section has no high-resistance layer, but has the same superjunction structure as the device section. Thus the decrease of breakdown voltage is likely to occur in the edge termination section because of variation in the amount of pillar dopant in the superjunction structure of the edge termination section.